(1) Field of the Invention
The present invention relates to surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices with unpolished nanocrystalline diamond which supports the surface acoustic wave. In particular the nanocrystalline diamond is composed of individual grains roughly between 1 nm and 50 nanometers. This material is also referred to as ultra-nanocrystalline diamond (UNCD).
(2) Description of Related Art
Bulk acoustic wave devices are also known. These find use as filters and in sensors. In general diamond is surrounded by one or more piezoelectric films or plates.
SAW devices function as transducers in that electromagnetic wave energy is converted to sound acoustic energy and then back to electrical wave energy at a particular frequency or frequencies, thus in a preferred use they act as filters. Surface acoustic wave (SAW) devices have found several key applications in radio frequency and microwave electronics (Campbell, C., Surface Acoustic Wave Devices and Their Signal Processing Applications, Academic Press, New York (1989)). They offer a high degree of frequency selectivity with low insertion loss making them highly suitable for use as narrow band filters. SAW devices are particularly well adapted to microwave integrated circuits since they can provide a significant size reduction over purely electromagnetic devices. This is a consequence of the small ratio of acoustic to electromagnetic wavelengths which, at a given frequency, is of order v/c ˜10−4, where v and c are the speeds of a surface acoustic wave and of light, respectively.
SAW devices are most typically implemented on piezoelectric substrates (quartz, lithium niobate) on which thin metal film interdigitated transducers (IDT) are fabricated using photolithography. The operating frequency f of a device is determined by the IDT period p which sets the wavelength of the surface wave λ. For example, a monolithic SAW device with IDT period 4 μm operates near 1 GHz for v˜4×103 m s−1 since f=v/λ. A simple IDT is typically composed of several finger pairs of length ˜100 λ with a finger width p/4. More complex designs with multiple pairs per wavelength and higher resonant frequencies tend to push the limits of conventional photolithography.
The use of diamond as a SAW substrate offers an attractive means for relaxing the lithographic criteria (Yamanouchi, K. et al., Proceedings IEEE Ultrasonics Symposium 351 (1989)). With a surface wave velocity v˜1×104 m s−1, diamond allows SAW device operation near 2.5 GHz with nominal 1 μm linewidth. Since diamond is not piezoelectric, additional complexity is introduced by a requisite overlayer of a piezoelectric thin-film, typically ZnO. Sound propagation in layered media may be highly dispersive and in general admits a multiplicity of allowed modes. Nevertheless, highly successful devices based on ZnO/polycrystalline diamond/Si layered structures have been reported, where the diamond layers have been grown by hot filament or CVD (chemical vapor deposition) techniques (Nakahata H., et al., Proceedings IEEE Ultrasonics symposium 377 (1992); Nakahata, Y. H. et al., ibid 361 (1995); Fujii, S., et al., ibid 183 (1997); Dreifus, K. L., et al., ibid 191 (1997); Nakahata, H., et al., ibid 319 (1998); and Hachigo, A., et al., ibid 325 (1995)).
Various U.S. patents which describe SAW devices are U.S. Pat. No. 6,051,063 to Tanabe et al., U.S. Pat. No. 5,814,149 to Shintani et al., U.S. Pat. No. 5,486,800 to Davenport, U.S. Pat. No. 5,773,911 to Tanaka et al., U.S. Pat. No. 5,920,143 to Tarui et al., and U.S. Pat. No. 6,127,768 to Stoner et al. which are incorporated herein by reference.
Nanocrystalline diamond (NCD) is a new form of diamond. See U.S. Pat. Nos. 5,989,511; 5,849,079; 5,772,760; 5,209,916 to Gruen; U.S. Pat. No. 5,328,676 to Gruen; U.S. Pat. No. 5,370,855 to Gruen; U.S. Pat. No. 5,462,776 to Gruen; U.S. Pat. No. 5,620,512 to Gruen; U.S. Pat. No. 5,571,577 to Zhang et al; U.S. Pat. No. 5,645,645 to Zhang et al; U.S. Pat. No. 5,897,924 to Ulczynski et al and U.S. Pat. No. 5,902,640 to Krauss. Its growth and characterization have been reviewed (Gruen, D. M., Annu. Rev. Mater. Sci. 29 211 (1999)). It differs from diamond-like carbon in that it contains relatively little hydrogen or sp2-bonded carbon. The grain size of NCD can be controlled by deposition conditions such as temperature, gas pressure, and hydrogen content. A distribution of grain sizes d is present in most deposits, with 1 nm<d<50 nm. As d decreases, an increasing fraction of the carbon content resides in or near grain boundaries.
For mechanical applications, the properties of NCD films that are most attractive are the naturally occurring smooth surfaces, a result of the small crystallite size, as well as its elastic isotropy for length scales >>d. These are quite relevant for SAW applications, since standard polycrystalline diamond on Si is quite rough and must be smoothed by mechanical polishing before photolithographic processing can be attempted.
Furthermore, one expects acoustic scattering at large angle grain boundaries in polycrystalline diamond, especially if lateral grain dimensions exist on length scales between acoustic wavelengths (˜1 μm) and SAW device apertures and transducer separations. The use of NCD would eliminate these concerns provided the elastic properties of NCD are not inferior to high-quality polycrystalline (or even single crystal) diamond.